The anti-wear (AW) and extreme pressure (EP) additives are mainly used for reducing friction and wear under boundary lubrication conditions. These additives are vital constituents of most lubricant formulations, under conditions of medium to high or extreme pressure, react with mating metal surfaces forming protective tribo-chemical layers. Thus the equipments are protected from wear and enabled to operate successfully under heavy loads.
Generally, any chemical constituent, pure or impure, intended or not, that is formed or deposited during lubrication on the metal surface, and able to separate and prevent the opposing surfaces from direct contact could theoretically be construed as AW/EP agent. Therefore, the classic AW/EP additives are oil-soluble chemicals or components which react with the metal surface forming a film that withstands both compression and, to a degree, shear. Since reaction with the metal is of the essence, only elements that can form iron compounds are truly eligible for this task. That makes compounds of sulfur, phosphorus, chlorine (or other halogens) preferential choices.
Traditionally, wear protection and friction modification by engine oil is provided by zinc dithiophosphate (ZDDP), molybdenum dithiophosphate (MoDDP) or other phosphorus compounds. These additives provide effective wear protection and friction control on engine parts through formation of a glassy polyphosphates anti-wear film. However, these additives may have one or more disadvantages such as;                1. copper and/or lead corrosion,        2. color darkening of the finished lubricant,        3. increased levels of sulfur and phosphorus in the finished lubricant.        
Among these disadvantages the level of phosphorus and sulfur in the engine oil is the most serious concern. This is because the deposition of phosphorus and sulfur species on automotive three way catalytic converters from lubricants has been known for some time to have a detrimental effect on poisoning the catalysts. Future generations of passenger car motor oils and heavy duty diesel engine oils require lower levels of phosphorus and sulfur in the finished oil in order to protect the pollution control devices. Hence the limits of phosphorus and sulfur levels in engine oil are reduced and supplemental forms of AW additives will be required to replace ZDDP. For example, current GF-4 motor oil specifications require a finished oil to contain less than 0.08 and 0.7 wt % phosphorus and sulfur, respectively and PC-10 motor oil specifications, the next generation heavy duty diesel engine oil, requires to contain less than 0.12 and 0.4 wt % phosphorus and sulfur, respectively. Certain molybdenum and organo zinc additives known in the industry contain phosphorus and sulfur at levels which reduce the effectiveness of pollution control devices.
Much work has gone in to reducing the level of ZDDP in lubricants by increasing the use of known friction modifiers, other phosphorus free components or balancing the properties of many compounds but this is difficult because ZDDP is not a mono-functional additive that provides only AW chemistry but has multifunctional properties providing anti-scuffing and anti-oxidation, all in one additive component.
In addition, it has complex interactions with other additives. Another approach is to modify the ZDDP molecule to have the same activity at lower concentrations by changing the alkyl group but a stable anti-wear ZDDP film cannot be formed by the modified ZDDP at low concentrations. Nevertheless, the ability to formulate with ashless dispersants would also benefit from the replacement of ZDDP chemistry. Ashless dispersants deteriorate the anti-wear performance of ZDDP because the amount of ZDDP adsorbed onto the metal surfaces is decreased by formation of complexes with ashless dispersants in oil. Therefore, lubricant additives and/or composition that delivers spectacular anti-friction and wear properties and as well as compatible with pollution control devices used for automotive and diesel engines are highly demanded.
Such lubricant additives and/or compositions compatible with pollution control devices should also not adversely affect oil solubility, corrosion and darkening the color of the finished lubricant. With rapid development of nano-science and technology, nanoparticles have received considerable attention in recent years because of their special physical and chemical properties. Especially in the field of tribology, many kinds of inorganic nanoparticles have been successfully used in lubricating oils and greases to solve wear and friction problems. The dispersion of inorganic nanoparticles in lubricating oil is still a principal problem for application of nanomaterial additives. In order to obtain better dispersion, a surface modification technique is usually adopted to structure an organic layer on the surface of nanoparticles. As compared to the conventional additives either containing heavy metals like Zn, Mo and Pb etc., or too much sulfur and phosphorus, greener nanomaterial additives with environmentally benign characteristics are strongly required.